(i) Field of the Invention
The present invention relates to a semiconductor manufacturing equipment, particularly to a wire bonding apparatus and method using at least ultrasonic energy as bonding means.
(ii) Description of the Prior Art
One of the many assembly processes for manufacturing a semiconductor device is what is known as the wire bonding process for connecting an electric terminal on a semiconductor chip with a lead terminal on a package of a lead frame or the like by a thin metallic wire. In this process, an end of the thin metallic wire is pressed against a bonding pad to bond it. The bonding is frequently performed by the ultrasonic method or ultrasonic thermocompression bonding method in addition to the thermocompression bonding method. A wire bonding apparatus and bonding process according to a general ultrasonic thermocompression bonding method are described below by referring to FIGS. 2 and 3.
As shown in FIG. 2, this type of wire bonding apparatus comprises an ultrasonic unit 31 for oscillation-controlling a high-frequency voltage, an ultrasonic vibrator 15 which is vibrated by a high-frequency voltage 24 supplied from the ultrasonic unit 31, a tool horn 32 for transmitting ultrasonic energy supplied from the ultrasonic vibrator 15, and a capillary 33 for transmitting ultrasonic energy to a thin metallic wire (hereafter referred to as a wire) 38 while guiding the wire 38. The tool horn 32 and capillary 33 move together vertically and horizontally so that an end of the wire 38 is accurately positioned on a bonding pad 36 of a semiconductor chip 35 to be bonded.
Steps of performing bonding according to a bonding method generally called ball stitch bonding by using the ultrasonic wire bonding apparatus with the above constitution are described below by referring to FIG. 3.
(1) An electric torch 51 is brought close to an end of the wire 38 to cause an electric discharge and to form a ball at the end of the wire 38 [FIG. 3(a)]. PA1 (2) When a ball 54 is formed, the electric torch 51 is moved to the original position [FIG. 3(b)]. PA1 (3) The capillary 33 is lowered, the ball 54 is made to contact a first bond electrode (chip pad) 36, and bonding is performed by crushing the ball with the end of the capillary 33 by means of pressure, heat, and ultrasonic energy [FIG. 3(c)]. PA1 (4) The capillary 33 is moved above a second bond electrode 58 and lowered. In this case, the wire 38 is sent out of the end of the capillary 33 to form a loop [FIG. 3(d)]. PA1 (5) The wire 38 is bonded with the second bond electrode 58 by means of pressure, heat, and ultrasonic energy and thereafter the capillary 33 is raised. Then, a clamp 56 is closed at a predetermined rising position and thereby a tension is applied to a joint and the wire 38 is cut [FIG. 3(e)].
In general, the wire 38 uses Au (gold), the chip pad (first bond electrode 36) uses Al (aluminum), and the lead frame electrode (second bond electrode 58) uses Ag (silver).
In the above steps, a mechanical contact resistance when the ball is bonded with the chip pad by the capillary (that is, load for ultrasonic vibration) is not always constant but it changes depending on, for example, the surface state of the pad, fusing characteristic of die bonding, or state of lead pressed. Therefore, a constant electric power is not transmitted to the joint due to a change of the load. Thus, ball removal due to incomplete bonding (or removal of the joint) may occur. To avoid the above trouble, it is necessary to enhance the bonding by supplying sufficient electric power. In this case, however, high density is difficult to obtain because a problem occurs that an extremely-crushed ball protrudes and contacts an adjacent pad.
The present invention is made to solve the above problem and its object is to provide a wire bonding apparatus realizing stable bonding by supplying a constant electric power to a joint.